My vote is for Pirateship, also. I can't say enough good things about them. But it is only available for US sellers. Foreign shipping is almost always cheaper than Etsy Labels, foreign packages track all the hops to the destination instead of just until it leaves the US, then nothing until it arrives, and the most important thing, if I have any questions or concerns, I can get a real person (not a bot) on chat and get an answer within 2 minutes.
When USPS sends bills that overcharge you, they take care of it and I always get refunded for those charges.
The problem with orders getting switched to Priority Mail on Etsy occurrs if you are using Calculated Shipping, because they have not entered Ground Advantage into their system after more than a year and they still have First Class Mail, also now defunct. With First Class, it automatically upgraded it to Priority if the weight was a pound or more, while the new Ground Advantage rate allows up to 70 pounds. You have to have Parcel Post or some class that doesn't kick it up if it is over a pound, or it wll always do this on heavier packages.
I don't use Etsy Labels so I don't know why it won't let you choose Ground Advantage when you actually go to buy your postage. It is not a problem on Pirateship.
Pirateship also has UPS at up to 80% discounts, which is usually cheaper for longer or heavier packages.
